Fatal error: Uncaught Error: Class "Drupal\serialization\Normalizer\FieldItemNormalizer" not found in /home/www/drupal8/test/core/modules/serialization/src/Normalizer/EntityReferenceFieldItemNormalizer.php:13 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/src/Normalizer/EntityReferenceFieldItemNormalizer.php on line 13